Abstract

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton
Gamma Ray Observatory has observed SN 1987A for two 2 week periods
during the first 9 months of the mission. Evidence for gamma-ray line
and continuum emission from Co-57 is observed with an intensity of about
10 exp -4 gamma/sq cm/s. This photon flux between 50 and 136 keV is
demonstrated by Monte Carlo calculations to be independent of the radial
distribution of Co-57 for models of low optical depth, viz., models
having photoelectric absorption losses of 122 keV photons no greater
than several percent. For such models the observed Co-57 flux indicates
that the ratio Ni-57/Ni-56 produced in the explosion was about 1.5 times
the solar system ratio of Fe-57/Fe-56. When compared with nearly
contemporaneous bolometric estimates of the luminosity for SN 1987A, our
observations imply that Co-57 radioactivity does not account for most of
the current luminosity of the supernova remnant in low optical depth
models. We suggest alternatives, including a large optical depth model
that is able to provide the SN 1987A luminosity and is consistent with
the OSSE flux. It requires a 57/56 production ratio about twice solar.

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